Abstract: Based on the mesomechanical damage theory and micro-void damage mechanism, a void growth ratio V_G was selected as the damage variable so as to establish an acoustic emission (AE) quantitative evaluation model. Carrying out tensile experiment on Q345 notched bar specimen obtained the AE information from a yield to fracture process. The meso-damage parameters distribution of a tensile specimen was simulated with the Gurson-Tvergaard- Needleman model, and their numerical solutions were achieved. Based on AE testing and Finite Element Simulation, the quantitative evaluation formula for Q345 between the void growth ratio V_G and AE cumulative hits N was advanced. The result shows that, in the tensile process of Q345 steel from yield to fracture damage, the function relationship of N and V_G can be divided into two stages, namely a linear damage stage and a nonlinear damage stage, and that when the N reaches 128, the material is at the transition state from a linear damage stage to a nonlinear damage stage.